=I 
34 The American Naturalist. [August, 
GEOLOGY AND PALEONTOLOGY. 
The Moon’s Face.—Mr. Gilbert’s address as retiring president 
of the Washington Philosophical Society is an ingenious array of 
arguments in favor of the impact theory to account for the origin of 
the features of the moon’s face. His hypothesis is, that material 
constituting the moon once surrounded the earth in the form of a 
aturnian ring; that the small bodies of this ring coalesced, first 
gathering around a large number of nuclei, and finally all uniting in a 
single sphere, the moon; that the lunar craters are the scars resulting 
from the collision of the moonlets. 
This hypothesis reconciles the impact theory with the circular out- 
line of the lunar craters, and explains the abundance of colliding 
bodies of large magnitude. The author discusses the probabilities of 
the formation, according to his theory, of lunar wreaths, central hills, 
arched inner plains, level inner plains,and the association of inner 
plains with central hills. He finds his theory adequate to explain all 
these phenomena, as well as the peculiarities known as furrows, sculp- 
ture, rills and rill pits. In regard to the “ white streaks” Mr. Gilbert 
quotes, as in accordance with his own idea, an unpublished suggestion 
made by Mr. William Wiirdeman, “ that a meteorite (moonlet) strik- 
ing the moon with great force spattered whitish matter in various 
directions. ” 
aaa the guis n: the mowi eap d ofthe moonlets must have colli- 
which have been obliterated 
Pe erosion and sedimentation. ` ET is possible the writer suggests, that 
these collisions imitated not only the differentiation of continental and 
oceanic plateaus, but the series of geographic transformations of which 
geologic structure is the record. (Phil. Soc. Washington, Bull. Vol. 
XII, 1893). 
North America during Cambrian Time.—Mr. Charles Wal- 
cott’s extensive knowledge of the Cambrian system of North America, 
‘has made it possible for him to reconstruct the form of the continent 
during that time. The land area is considered at the inception of 
Cambrian time, and its history is traced in a broad manner to the 
closing epoch of the period. 
By a form of deductive reasoning from the mode of sedimentation 
‘the author first determines an approximate shore line of the ancient 
